Ballasts for gas discharge lamps are often classified into two groups according to how the lamps are ignited—preheat and instant start. In preheat ballasts, the lamp filaments are preheated at a relatively high level (e.g., 7 volts peak) for a limited period of time (e.g., one second or less) before a moderately high voltage (e.g., 500 volts peak) is applied across the lamp in order to ignite the lamp. In instant start ballasts, the lamp filaments are not preheated, so a higher starting voltage (e.g., 1000 volts peak) is required in order to ignite the lamp. It is generally acknowledged that instant start operation offers certain advantages, such as the ability to ignite the lamp at a lower ambient temperatures and greater energy efficiency (i.e., light output per watt) due to no expenditure of power on filament heating during normal operation of the lamp. On the other hand, instant start operation usually results in considerably lower lamp life than preheat operation.
Because a substantial amount of power is unnecessarily expended on heating the lamp filaments during normal operation of the lamp, it is desirable to have preheat-type ballasts in which filament power is minimized or eliminated once the lamp has ignited. Ballasts that provide filament preheating prior to lamp ignition, but that cease to provide filament heating after the lamp ignites, are commonly referred to as programmed start ballasts.
When a lamp is operated at a current level that approaches the rated normal operating current of the lamp (e.g., about 180 milliamperes rms for a T8 lamp), the absence of filament heating has little negative impact upon the useful operating life of the lamp. Thus, ordinary programmed start ballasts work well with lamps that are driven at a normal (i.e., full-light) level. Conversely, when a lamp is operated at a current level that is substantially less than the rated normal operating current of the lamp (i.e., such as what occurs when the lamp is operated in a dimmed mode), the absence of filament heating has been observed to have a considerable negative impact upon the useful operating life of the lamp. Thus, ordinary programmed start ballasts are not well suited for driving lamps at substantially reduced light levels.
Therefore, a need exists for a ballast that primarily operates in a programmed start manner (i.e., that provides filament heating prior to lamp ignition, and then no filament heating during full-light operation of the lamp), but that has an added feature of providing filament heating during dimmed operation of the lamp. Such a ballast would represent a significant advance over the prior art.